The present invention relates to an improved manifold-nozzle assembly for use in an injection molding machine for manufacturing molded plastic articles, which assembly includes a clamp ring arrangement for clamping a nozzle housing against a manifold while permitting the nozzle housing to slide against the manifold as a result of thermal expansion.
There are numerous designs for injection molding nozzle assemblies which utilize the concept of the nozzle back face being free to slide against the front surface of a hot runner manifold in communication with the nozzle housing. In these designs, it is necessary to apply a high force to keep the nozzle and manifold tightly together and to preclude plastic leakage between the two as a result of the plastic injection pressure during the molding process.
U.S. Pat. No. 4,588,367 to Schad illustrates a system wherein a spring arrangement for pushing the nozzle against the manifold is used to provide plastic sealing pressure. This arrangement works well for smaller nozzles having relatively small melt channels which do not create large separating forces between the nozzle and the manifold. However, as larger nozzles are required for the molding of larger plastic parts, the separation forces become much greater due to the increased projected area of the larger melt channel subjected to the pressure of the plastic melt. A satisfactory spring design for large nozzle systems would be prohibitively large and cumbersome. Still further, in a spring system, the mold plates must be precision machined to ensure that the spring pressures are sufficient and consistent. This is a problem in the manufacturing process because metal cutting machines used to fabricate large mold parts are conventionally not as accurate or precise as smaller machines and hence desired dimensional tolerances are exceedingly difficult and time consuming to satisfy. If tolerances are not met, the result can be insufficient clamping force on the nozzle to prevent plastic leakage or possible overclamping forces which can damage the hot runner components and also cause leakage.
U.S. Pat. No. 4,981,431 to Schmidt illustrates a nozzle which is directly screwed onto a manifold. Threads are provided in the back face of the nozzle and screws passing through the manifold hold the nozzle tightly to it, providing a preload to aid against plastic leakage at the melt channel interface. This design incorporates a nozzle flange and opposing pressure pad which act to squeeze the manifold and nozzle housing together during thermal expansion. As shown in this patent, the nozzle is centered and axially located in the mold plate while the manifold is allowed to expand as it heats up to operating temperature. The expansion of the manifold causes sliding between the manifold front face and the nozzle back face. The disadvantage of this approach is that the fastening screw head will travel with the manifold expansion while the screw thread will remain fixed in the nozzle which is restricted from lateral movement. The result is that the screw can be excessively bent and stretched causing immediate failure or loss of sufficient properties to maintain the interfacial plastic seal under injection pressure. This is especially true if the manifold is long or large in size and dictates a greater thermal expansion distance to travel, causing much greater stresses to be imposed upon the nozzle screws.
U.S. Pat. No. 4,832,593 to Brown shows a method to isolate the screws from travelling while still providing a clamping or sealing force between the nozzle and manifold. A manifold cap provides a resting surface for the screw heads and is keyed to a nozzle extension so that neither the screw head nor the screw thread is required to move laterally during thermal expansion of the manifold. This design can be overly bulky as it must reach out over and around the hot runner manifold to grip it like a vise. Also, freedom of manifold design is not permitted in the area directly over the nozzle as it must be sized and shaped to receive the nozzle extension and the manifold cap.
U.S. Pat. No. 4,793,795 to Schmidt et al. shows a method of locating a nozzle spigot in the side of a manifold and holding it against the manifold using arcuately shaped members fastened with screws through the manifold. This design has a number of shortcomings. First, as the manifold expands, it carries the nozzle spigot and hence the rear of the nozzle with it. This results in the nozzle tipping axially as the rear of the nozzle moves with the manifold while the front of the nozzle stays located at its front by the tip insert in the mold plate. Excessive wear and pressure are put on the tip as it must resist the forces acting on the rear portion of the nozzle. Also, as the manifold expands in all directions from the locating dowel, the nozzle will actually be lifted as well as pushed laterally. Because the nozzle front portion forms part of the molding surface, it will be necessary to perform complex calculations to determine how much movement will be observed at the nozzle tip. The tip design will need to compensate for all movements caused axially and laterally, due to thermal expansion of the manifold. Additionally, the nozzle is more difficult to manufacture with a side mounting spigot feature and the manifold is also more expensive to make, needing a mating counterbore to receive the nozzle spigot.
There remains a need for an effective nozzle-manifold assembly which compensates for thermal expansion while providing an effective seal between the nozzle housing and the manifold.